1. Field
The present disclosure relates generally to manufacturing composite structures and, in particular, to devices used for manufacturing composite structures. Still more particularly, the present disclosure relates to a method and apparatus for fabricating an elastomeric bladder for manufacturing composite structures.
2. Background
Aircraft are being designed and manufactured with greater and greater percentages of composite materials. Composite materials are used in aircraft to decrease the weight of the aircraft. This decreased weight may improve performance features such as, for example, without limitation, payload capacity and fuel efficiency. Further, composite materials may provide longer service life for various components in an aircraft.
Composite materials may be tough, light-weight materials created by combining two or more functional components. For example, without limitation, a composite material may include reinforcing fibers bound in a polymer resin matrix. The fibers may be unidirectional or may take the form of a woven cloth or fabric. The fibers and resins may be arranged and cured to form a composite structure.
Using composite materials to create aerospace composite structures may allow for portions of an aircraft to be manufactured in larger pieces or sections. For example, without limitation, a fuselage in an aircraft may be created in cylindrical sections. Other examples may include, without limitation, wing sections joined to form a wing or stabilizer sections joined to form a stabilizer.
In manufacturing composite structures, layers of composite material may be laid up on a tool. The layers of composite material may be comprised of fibers in sheets. These layers may take the form of, for example, without limitation, fabrics, tape, tows, and/or other suitable configurations for the sheets. In some cases, resin may be infused or pre-impregnated into the sheets. These types of sheets are commonly referred to as prepreg.
The different layers of prepreg may be laid up in different orientations and different numbers of layers may be used depending on the desired thickness of the composite structure being manufactured. These layers may be laid up by hand and/or by using automated lamination equipment such as a tape laminating machine or a fiber placement system.
In laying up the layers of prepreg, composite structures in the form of panels may be defined. For example, without limitation, the layers of prepreg may be laid up to form a skin panel. The skin panel may have a thickness that is selected to reduce the weight of the aircraft. When the skin panel is light and thin, the skin panel may be more flexible than desired.
Stiffening structures may be included with the skin panel to reduce flexing, vibrations, and/or other undesirable movement. A stringer is an example of a stiffening structure that may be formed with the skin panel to reduce the flexing, vibrations, and/or other undesirable movement in the skin panel during use. Additional layers of prepreg also may be laid up to form stringers for the skin panel.
In forming a stringer, layers of prepreg are laid up in a desired shape for the stringer. A stringer may have a channel. The channel may be defined in the stringer using a mandrel. The mandrel may take the form of an elastomeric bladder.
The layers of prepreg may be laid up on a tool in the shape of a stringer, with the elastomeric bladder defining the channel of the stringer within the layers of prepreg, in an uncured form. After the different layers of prepreg have been laid up on the tool, the layers of prepreg may be consolidated and cured upon exposure to temperature and pressure, thus forming the final composite structure in a cured form. The elastomeric bladder may be inflated to support the surfaces of the stringer during the fabrication process.
In inflating the elastomeric bladder, the elastomeric bladder may be sealed. The seal may be made with respect to the tool, a vacuum bag used to apply pressure to the layers of prepreg, or some combination thereof. Currently, the elastomeric bladder may be sealed using a sealant tape that seals the elastomeric bladder to the vacuum bag. This technique, however, may not be as reliable as desired and may be labor-intensive. Further, inconsistencies may develop in the elastomeric bladder that may cause the elastomeric bladder to perform in an undesired manner.
One solution may involve having a metallic fitting at an end of a pre-cured elastomeric bladder. For example, without limitation, the elastomeric bladder may be first fabricated and cured. The metal fitting may then be bonded to an interior surface within a cavity of the cured elastomeric bladder. When the metal fitting is bonded to the interior surface of the bladder, the expansion of the bladder may pull the bladder away from the metal fitting.
This separation may occur during the curing of a stringer using the bladder, which may result in an undesired shape for the stringer. The separation also may result in a leak between the bladder and the end of the fitting that reduces the pressure applied to the stringer on a skin panel. Consequently, the skin panel may need to be reworked, discarded, or some combination thereof. Therefore, it would be desirable to have a method and apparatus that takes into account at least some of the issues discussed above, as well as other possible issues.